Hi Mark, welcome.
First of all, hopefully you routed the belts correctly. If not, it obviously won't work right. That being said....
Many boomboxes have a pivoting idler wheel that mounts on a fulcrum which flips the wheel from one side to the other depending on whether FF or RW function. If that lever is not free, it could be stuck in one position. Maybe look into that. Typically, when you service belts, it's equally important to clean off all old grease, grime, dust furbees, and ensure that all levers, pivots, slides, etc are free.